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<div class="titlepage"><div><div><h3 class="title">
<a name="phoenix.inside.custom_terminals"></a><a class="link" href="custom_terminals.html" title="Custom Terminals">Custom Terminals</a>
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<p>
        Custom Terminals are used in Phoenix to handle special values transparently.
        For example, as Phoenix captures everything by value, we needed to use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span></code> to bring reference semantics
        into Phoenix.
      </p>
<p>
        Custom terminals could be any wrapper class:
      </p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">is_custom_terminal</span><span class="special">;</span>
</pre>
<p>
        needs to be specialized in order for Phoenix to recognize this wrapper type.
        <code class="computeroutput"><span class="identifier">default_action</span></code> calls <code class="computeroutput"><span class="identifier">custom_terminal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>.
      </p>
<p>
        Example:
      </p>
<pre class="programlisting"><span class="comment">// Call out boost::reference_wrapper for special handling</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">is_custom_terminal</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;</span>
  <span class="special">:</span> <span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span>
<span class="special">{};</span>

<span class="comment">// Special handling for boost::reference_wrapper</span>
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">custom_terminal</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="special">&gt;</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">&amp;</span><span class="identifier">result_type</span><span class="special">;</span>

    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">&gt;</span>
    <span class="identifier">T</span> <span class="special">&amp;</span><span class="keyword">operator</span><span class="special">()(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">r</span><span class="special">,</span> <span class="identifier">Context</span> <span class="special">&amp;)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">r</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
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